In a conventional zigzag sewing machine, it a needle dropping position is different as shown in FIG. 1 as in a left basic line (L), a middle basic line (M) and a right basic line (R), crossing conditions between a needle 1 and a beak 2 of a loop taker are differed respectively, differently from straight stitching sewing machines. If a size of a thread loop of the middle basic line (M) is made proper, the crossing conditions are worst at the remotest needle dropping positions of the left (L) and the right (R). A loop of the upper thread is too large in the left (L), and the loop is too small in the right (R). Thus, the conditions are contrary to each other, and hooking of the thread loop is difficult in each of the needle dropping positions causing skipping or breaking the thread.
In order to improve such crossing conditions, it was proposed to make the vertical movement of the needle slow when hooking the thread loop, or to increase the speed of rotational movement of the loop taker for removing the causes of skipping or breaking of the thread. Unfortunately, such ideas have not been yet commercialized.